Aircrafts are often operated in contaminant-prone environments, including in desert areas. The contaminant-prone environments, coupled with levels of maintenance that are practiced by particular airlines, often lead to accumulation and build-up of contamination on the narrowest passages of environmental control systems (ECSs), namely and often heat exchangers in the ECSs. Accumulated contamination may lead to reduction of performance over time, and in some cases may lead to failures that cause aircraft-on-ground (AOG), customer dissatisfaction, and elevated repair costs. Airframers and airlines in general prefer to avoid fixed maintenance cleaning intervals, instead preferring on-condition maintenance due to the high cost of removing and cleaning the ECSs at fixed intervals. In addition, on-wing cleaning is not often very practical.
Accumulation of contamination, also referred to as fouling, in heat exchangers of the ECSs has been investigated in the past. In particular, past investigations have focused upon causes of fouling and methods to avoid or mitigate such fouling. Other efforts have focused on detecting fouling in the heat exchangers of the ECSs, with appropriate steps taken to clean or provide notification of excessive fouling after detection. Conventional methods for detecting fouling in the heat exchangers of the ECSs include:    1. Examination of the heat transfer coefficient or heat conductance,    2. Simultaneous observations of pressure drops and mass flow rates,    3. Conducting temperature measurements,    4. Ultrasonic or electrical measurements,    5. Weighing of heat exchanger plates, and    6. Modeling the heat exchanger and comparing the prediction with filed data.
The conventional methods for detecting fouling all are prone to drawbacks. For example, to be sufficiently accurate, the conventional methods 1-3 require that the heat exchangers present successive steady states, i.e., the inlet temperatures and flows must be stable for a period long enough to be able to compute or measure the values of interest. Conventional method 4 is a local method limited to portions of the heat exchangers and cannot be generally applied to the heat exchangers as a whole. Conventional method 5 requires that the process be stopped, with the heat exchangers being disassembled for measurement. Such requirements are too restrictive or costly. Conventional method 6 is reliant upon the quality and fidelity of the mathematical modeling itself. For example, a Diagnostics, Prognostics and Health Management (DPHM) solution is known for aircraft ECS heat exchangers to detect on-line fouling by use of a dynamic non-linear mathematical model. The model parameters are a function of mass flow rates and core and fin temperatures. Measurements of the inlet and outlet temperatures and the mass flow rates are used for model parameter estimation with Extended Kalman Filtering (EKF). However, not all values that are employed in conventional mathematical modeling are available via measurements within conventional heat exchangers, and often require invasive sensing equipment. For example, mass flow rates and heat capacities are difficult to measure and require the invasive sensing equipment.
Accordingly, it is desirable develop new techniques for contamination monitoring of heat exchangers that could alert a maintenance crew in advance to prepare for a timely removal or cleaning of the heat exchangers and thereby minimize disruption when contamination levels exceed acceptable thresholds. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.